1. Field of the Invention
The present invention relates to a call admission control device, and call admission control method, and more specifically to a call admission control device and a call admission control method for controlling the admission of a call in a mobile communication system where dedicated channels and shared channels are mixed.
2. Description of the Related Art
A mobile telephone communication system performs a communication using finite resources (frequencies and power), and there is the upper limit to the communication capacity. Therefore, it is necessary to limit the number of mobile stations in a cell depending on the communication capacity. Practically, when a new mobile station starts communications in the cell, it is necessary to judge whether or not the new mobile station can start a communication in the cell. The control is referred to as call admission control. The status in which a new mobile station cannot start a communication in the cell, that is, the status in which the communication capacity is being used substantially 100%, is referred to as a capacity limit.
The call admission control in the W-CDMA (wideband-code division multiple access) of the third generation mobile communication system is explained below.
In the release 99 system of the W-CDMA, a one-to-one communication channel which is called a dedicated channel is set between a radio base station and a mobile station, and communications are performed using the dedicated channel. In this case, with an increasing number of mobile stations performing communication with the radio base station, consumed power resources, code resources, etc. increase. Thus, in the release 99 system in the W-CDMA system, call admission control is performed normally based on power resources, code resources, and baseband resources, etc. in a radio base station.
Described below as an example is a case in which the call admission control is performed based on the power resources. For example, 43 dBm is set as the maximum transmission power of a radio base station, and 42.5 dBm is set as a threshold value for call admission. The difference of 0.5 dB is a margin for keeping the transmission power of a radio base station below the maximum transmission power. When there is a mobile station which is newly starting communications in a corresponding cell, and if the transmission power of a radio base station at the time point is 40 dBm, then there are sufficient power resources. Therefore, the communication of the mobile station is admitted. On the other hand, if there is a mobile station which is newly starting communications in a corresponding cell, and if the transmission power of a radio base station at the time point is 42.6 dBm, then it is determined there are not sufficient power resources, and the communication of the mobile station is not admitted. The transmission power of above-mentioned radio base station is constituted by, for example, the transmission power of common channels such as a CPICH (common pilot channel), a PCCPCH (primary common control physical channel), SCH, etc., the transmission power of a dedicated channel set between a radio base station and each mobile station, etc.
On the other hand, in the release 5 system of the W-CDMA, in addition to a dedicated channel set one to one between a radio base station and a mobile station, one or more than two shared channels is set for a plurality of mobile stations that share one large channel. The communication system using the shared channels is called high speed downlink packet access (HSDPA), and is a downlink high speed packet transmission system capable of transmitting high-speed and large-capacity traffic (refer to 3GPP TR25.848v4.0.0, for example). That is, the release 5 system in the W-CDMA system contains in a mixed manner the above-mentioned communication using shared channels and communication using dedicated channels.
In the above-mentioned system containing in a mixed manner a communication using dedicated channels and a communication using shared channels, it is necessary to share radio resources such as power resources and code resources between the dedicated channels and the shared channels.
For example, there can be a method of equally dividing available radio resources such as power resources, code resources, etc., and allocating the divided resources to the dedicated channels and the shared channels.
There is also a method of first allocating radio resources such as power resources, code resources, etc. to dedicated channels, and then allocating the remaining radio resources to shared channels.
Since the communication system using shared channels performs communications using available radio resources as much as possible, the radio resource can be more efficiently applied in the latter example in the above-mentioned two examples.
However, in the above-mentioned method of allocating radio resources such as power resources, code resource, etc. first to dedicated channels, and then allocating remaining radio resources to shared channels, there is a problem that the radio resources to be allocated to the shared channels are reduced when there is an increasing number of mobile stations for performing communications using the dedicated channels.
JP2005-525743A (hereinafter referred to as patent document 1) describes the technique of managing radio resources relating to the communications performed using dedicated channels and high-speed shared channels. JP2003-143653A (hereinafter referred to as patent document 2) describes the technique of controlling the transmission data rate of shared channels when the increase or decrease of the number of dedicated channels is detected.
As described above, there is a method of performing call admission control based on the remaining amount of radio resources such as power resources, code resource, etc. as a method of controlling call admission in a mobile communication system containing in a mixed manner dedicated channels and shared channels. As a method of allocating the radio resources, there is a method of first allocating the radio resources to dedicated channels, and then allocating the remaining radio resources to shared channels.
However, in the conventional call admission control method, there is the problem that radio resources to be allocated to shared channels are reduced when there is an increasing number of mobile stations performing communications using dedicated channels. When the radio resources to be allocated to the shared channels are considerably reduced, the transmission data rate of the mobile stations performing communications using the shared channels is conspicuously reduced, which is not desired from the viewpoint of services. The problem cannot be solved in the techniques described in the patent document 1 or 2.